MAX8734AEEI+ Maxim Integrated Products, MAX8734AEEI+ Datasheet - Page 25

• Manufacturer Part Number: MAX8734AEEI+
• Description: IC PWR SUPPLY CONTROLLER 28QSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAX8734AEEI+

• Applications: Power Supply Controller
• Voltage - Input: 4.5 ~ 24 V
• Current - Supply: 25µA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-QSOP
• Product: Power Monitors
• Operating Temperature Range: - 40 C to + 85 C
• Mounting Style: SMD/SMT
• Accuracy: 1.5 %
• Supply Current (max): 50 uA
• Supply Voltage (max): 4.5 V
• Supply Voltage (min): 24 V
• Case: SSOP
• Dc: 06+
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The MAX8732A/MAX8733A/MAX8734As’ pulse-skip-
ping algorithm (SKIP = GND) initiates skip mode at the
critical conduction point, so the inductor’s operating
point also determines the load current at which
PWM/PFM switchover occurs. The optimum point is
usually found between 20% and 50% ripple current.
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as follows:
Example: I
200kHz, 35% ripple current or LIR = 0.35:
Find a low-loss inductor with the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice. The core must be large enough
not to saturate at the peak inductor current (I
The inductor ripple current also impacts transient-
response performance, especially at low V+ - V
differences. Low inductor values allow the inductor cur-
rent to slew faster, replenishing charge removed from
the output filter capacitors by a sudden load step. The
peak amplitude of the output transient (V
function of the maximum duty factor, which can be cal-
culated from the on-time and minimum off-time:
where minimum off-time = 0.350µs (max) and K is from
Table 2.
The minimum current-limit threshold must be great
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The val-
V
SAG
at maximum load).Inductor values lower than this
grant no further size-reduction benefit.
I
PEAK
=
L
2
=
×
Supply Controllers for Notebook Computers
LOAD(MAX)
12
(
= I
∆
L
C
I
High-Efficiency, Quad-Output, Main Power-
V
LOAD MAX
OUT
LOAD(MAX)
=
×
Determining the Current Limit
V
200
×
______________________________________________________________________________________
(
+ × ×
5 12
V
V
V
OUT
OUT_
(
kHz
= 5A, V+ = 12V, V
f
)
)
2
V
_
+ [(LIR / 2) x I
LIR
×




(
−
×
K
V
5

 

L K
0 35
+ −
V
V
×

 

.
)
+ −
Inductor Selection
I
LOAD(MAX)
V
V
OUT
OUT_
×
V
V
V
+
+
OUT
5
_
A
LOAD(MAX)
)
+
=
_
OUT5
SAG
t
8 3

  −

OFF MIN
PEAK
. µ
t
) is also a
(
OFF MIN
H
= 5V, f =
):
(
)
]

 

OUT_
)




ley of the inductor current occurs at I
half of the ripple current; therefore:
where I
voltage divided by the R
For the MAX8732A/MAX8733A/MAX8734A, the mini-
mum current-limit threshold voltage is 93mV (ILIM_ =
V
from the MOSFET N2/N4 data sheet and add some
margin for the rise in R
general rule is to allow 0.5% additional resistance for
each °C of temperature rise.
Examining the 5A circuit example with a maximum
R
7.75A is greater than the valley current of 4.125A, so
the circuit can easily deliver the full-rated 5A using the
fixed 100mV nominal current-limit threshold voltage.
Connect the source of the synchronous rectifier to a
current-sense resistor to GND (MAX8732A/MAX8733A),
and connect CS_ to that junction to set the current limit
for the device. The MAX8732A/MAX8733A/MAX8734A
limit the current with the sense resistor instead of the
R
resistor can be calculated with the equation:
The output filter capacitor must have low enough equiv-
alent series resistance (ESR) to meet output ripple and
load-transient requirements, yet have high enough ESR
to satisfy stability requirements. The output capaci-
tance must also be high enough to absorb the inductor
energy while transitioning from full-load to no-load con-
ditions without tripping the overvoltage fault latch. In
applications where the output is subject to large load
transients, the output capacitor’s size depends on how
much ESR is needed to prevent the output from dip-
ping too low under a load transient. Ignoring the sag
due to finite capacitance:
where V
drop. In non-CPU applications, the output capacitor’s
size depends on how much ESR is needed to maintain
an acceptable level of output voltage ripple:
CC
DS(ON)
DS(ON)
I
LIMIT(LOW)
). Use the worst-case maximum value for R
I
LIMIT(LOW)
LIMIT(LOW)
DIP
= 12mΩ at high temperature reveals the following:
of N2/N4. The maximum value of the sense
is the maximum-tolerable transient voltage
> I
= 93mV / 12mΩ > 5A - (0.35 / 2) 5A
I
LOAD(MAX)
LIM_
7.75A > 4.125A
R
= minimum current-limit threshold
ESR
Output-Capacitor Selection
= 93mV / R
DS(ON)
≤
DS(ON)
I
LOAD MAX
- [(LIR / 2) x I
V
with temperature. A good
DIP
(
of N2/N4 (MAX8734A).
SENSE
)
LOAD(MAX)
LOAD(MAX)
DS(ON)
minus
]
25